cephfs client: add zero-copy capability

cephfs client: add zero-copy capability

For writes, the incoming iovec buffers are pushed into the bufferlist.

For reads, an incoming iovec is used to communicate the desired read
length, but then on read complete, an iovec would be built from the
bufferlist and passed back to the client.

Signed-off-by: Frank S. Filz <ffilzlnx@mindspring.com>